The present invention generally relates to vacuum pumps, and more particularly, to pumps known as turbomolecular pumps characterized by "bladed" rotor and stator construction with running clearances in the millimeter range which are particularly effective in the free molecular flow range. More particularly, this invention is directed to an improved bearing system especially useful for vacuum pumps of the single ended type which have a bearing between the motor rotor and the pump rotor discs. In this regard, an important aspect of the present invention concerns a bearing assembly for a single ended turbomolecular pump which is characterized by improved heat transmission capabilities and a high tolerance for misalignment whereby noise, vibration and wear are minimized and longer bearing life is achieved.
In conventional single ended turbomolecular vacuum pumps, two vertically disposed bearings are commonly employed to support the rotating system with the mass center of the rotating system usually being concentrated at the upper bearing. Maintaining stability in rotating machines of this type requires great care due to the very high rotational speeds which are utilized in these machines. Replacing worn bearings in these machines involves the disassembly of the rotating system which by itself is a laborious and time consuming process. Furthermore, because the natural spin axis is slightly different with each bearing replacement reassembly, the rotor system can never be in perfect balance on its bearings. The bearings must be allowed to "float" in free vibration. A sufficiently compliant bearing mount is therefore required for bearing replacement by the user without rebalancing facilities.
In addition to supporting the rotating system, bearings also serve to conduct heat away from the rotor. Most commercially available turbomolecular pumps employ rotors of aluminum alloy which are subject to loss of strength starting at about 148.degree. C. Since turbolecular pump rotors operate in a thermally insulating vacuum, the only way the rotor can be cooled, without unduly complicating the system, from common heat input such as bearing friction, integral motor losses and gas friction, is by conduction through the bearing lubricant and by radiation.
Heretofore, the majority of turbomolecular vacuum pumps have utilized oil lubricated ball bearings. Other types of bearings such as air film bearings and magnetic bearings have also been employed but have not achieved widespread popularity for a variety of reasons. For instance, the low stiffness provided by magnetic bearings and the complexity of the electronic controls necessary to maintain them result in operational limitations and reliability problems.
Ball bearings are well adapted to the high speed application of turbomolecular pumps because their friction increases only moderately as the speed increases, and they do not generally develop internal instability with increasing speed. Furthermore, they offer a great reserve of strength and rigidity to handle extraordinary air inrush thrust loads. These bearings are located at the discharge side of the rotor disc cascade, and oil vapors associated therewith do not penetrate upstream to the turbo inlet.
However, while the reliability of the ball bearings currently available is very high, the balls orbiting around the axis at very high angular velocity cause substantial contact forces between the balls and raceway which are far greater than those imposed by the rotor alone. Misalignment of the bearing raceways relative to the rotor axis converts these forces to noise, vibration and wear which eventually requires replacement of the bearing. Precise machining and meticulous care in handling and assembling the parts are, therefore, a necessity to minimize misalignment.
In addition to the handling and assembly constraints, the heat transmitting capabilities of ball bearings limit the effective operating range of a turbomolecular pump. Two principal sources of heat input to a turbomolecular pump rotor system are oil friction and gas friction. The gas friction heat input is proportional to the pressure range in which the rotor is operating, i.e. lower gas friction at lower pressures and higher gas friction at higher pressures. The heat generated from gas friction is usually dissipated by the flow of oil through the bearing supporting the rotor. However, the throughput of oil in a typical ball bearing assembly is proportional to both its heat transmission capability and the amount of friction generated by the oil itself.
Therefore, when gas friction heat input is negligible, a ball bearing with minimum oil is best while when gas friction heat input is high maximum oil is best. However, the concommittant effect of increased oil friction with increased oil throughout eventually produces a counter-balancing effect to the heat transmission capability of the bearing which limits the high pressure end of the useful range of the pump and thereby curtails the use of turbomolecular pumps in the higher pressure range often needed for sputter and dry etch applications used in integrated circuit manufacture.
The present invention solves the aforementioned problems by providing an improved bearing assembly of the "plain" or "sleeve" liquid lubricated type which can tolerate far more misalignment than a typical ball bearing without increase in noise, wear, or fatigue damage and which has better heat transmission capabilities than a typical ball bearing. Heretofore such bearings have not been employed in turbomolecular pumps because such bearings having a conventional length to diameter ratio would impose far too much fluid friction drag at the speeds employed in such a turbo pump. In addition to fluid friction drag, oil film bearings are prone to exhibit a hydrodynamic instability known as "whirl" which can produce intolerable vibrations. It has been discovered that when the ratio of journal diameter to bearing length is relatively high, 10:3 or above, the above noted disadvantages are diminished to the point where the friction drag encountered is less than that obtained with a ball bearing of comparable diameter, and a lubricant of comparable viscosity and where there is no appreciable whirl. In addition, this new bearing assembly has the potential for unlimited life and therefore avoids the balancing problems of bearing replacement reassembly.
It is, therefore, a general object of the present invention to provide an improved turbomolecular pump.
Another object of the present invention is to provide a turbomolecular pump with an improved bearing assembly which substantially reduces noise, wear or fatigue damage.
Another object of the present invention is to provide a turbomolecular pump with an improved bearing assembly which has improved heat transmission capabilities thereby extending the useful high pressure range of the pump.
Another object of the present invention is to provide an improved turbomolecular pump of the single ended type which incorporates an oil film bearing assembly.